This invention relates to electrostatic spraying.
Our UK specification No. 1 569 707 discloses an electrostatic spraying apparatus wherein a sprayhead has a conducting or semiconducting surface which is charged to a potential of the order of 1 to 20 Kilovolts and a field intensifying electrode which is mounted adjacent to the surface and is connected to earth (ground) potential. When spraying liquid is delivered to the sprayhead, the electrostatic field at the surface is sufficient to cause liquid to be atomized without substantial corona discharge. Charged particles of liquid emerging from the sprayhead are projected past the electrode to a target, which is also at earth potential.
The provision of the earthed field intensifying electrode offers three advantages. First, the electrostatic field at the conducting or semiconducting surface is greater than it would otherwise be, since the electrode is much closer to the surface than is the target. This means that the potential applied to the surface can be lower, which means that a cheaper and safer generator can be employed. Secondly, the spacing between the electrode and the conducting or semiconducting surface, and hence the electrostatic field at the surface, is constant. In spraying operations which involve movement of a sprayhead relative to a target, such as crop spraying, there can be major variations in the spacing between the sprayhead and the target. If there is no field intensifying electrode, such variations in spacing cause corresponding variations in the effective electrostatic field. Finally, in spraying operations which produce small, satellite droplets of spraying liquid, such smaller particles can be attracted to the field intensifying electrode.
In large scale agricultural spraying there is a continual demand for apparatus capable of operating at higher flow rates and there is also a demand for smaller droplet size, for example, down to approximately 30 .mu.m diameter. These demands are conflicting, since increasing the flowrate produces an increase in the size of the droplets, other parameters remaining constant. Moreover, the combination of a high flowrate and a small droplet size causes a large "back spray" of droplets, which are repelled away from the main body of droplets and settle on the apparatus or drift away into the air.